Fruit and nut trees, as well as bushes, shrubs, vines and berries, require winter chilling to grow normally. The amount of chilling required depends upon the kind of fruit and the cultivar. If winter chilling is insufficient, then growth abnormalities such as delayed and uneven blossoming, poor leaf cover, insufficient fruit-set and reduced fruit size can occur. These symptoms are generally referred to as delayed foliation according to the article, Blommaert, K. L. J., "Winter Dormancy and Delayed Foliation," The Deciduous Fruit Grower, (1956).
Measures to reduce the symptoms of delayed foliation include treatment with high volumes of chemical rest-breaking agents during later winter and various physical manipulations such as pruning.
In South Africa, for example, most apple trees receive insufficient winter chilling to break rest completely and thus annual application of a chemical rest-breaking agent is standard practice. The most widely used rest-breaking agent in commercial apple orchards is dinitro-ortho-cresol (DNOC) in combination with winter-oil emulsion (a lipophilic agent) (Erez, A. et al., "Improved Methods of Breaking Rest in the Peach and Other Deciduous Fruit Species," J. Amer. Soc. Hort. Sci., 96, pp. 519-522 (1971)). This article also mentions the use of the potassium salt of gibberellic acid, kinetin, indoleacetic acid and thiourea as potential rest-breaking agents.
DNOC is a non-systemic stomach poison and contact insecticide, ovicidal to the eggs of certain insects. It is strongly phytotoxic and its permissable use as an insecticide is limited to dormant sprays or on waste ground. Further, DNOC is known to act as a cumulative poison in man. Although DNOC is relatively inexpensive and effective as a rest-breaking agent, it is toxic enough to limit its continued use and it is currently on the European Red List of agricultural chemicals which will be prohibited in the future. DNOC is also banned in the United States for use as an agricultural chemical.
One of the most effective alternatives to DNOC is hydrogen cyanamide according to the publications, Snir, I., "Chemical Dormancy Breaking of Red Raspberry," HortScience, 18, pp. 719-713 (1983); North, M. S., "Effects of Cyanamide and DNOC/oil on Budbreak, Yield and Fruit Size of Golden Delicious Apples," S. Afr. J. Plant Soil, 6(3), pp. 176-178 (1989); Stadler, J. D., North, M. S. and Lutze, G. F. A., "Artificial Rest-Breaking of Apricot and Plum Cultivars Using Hydrogen Cyanamide," J.S.Afr.Soc.Hort.Sci., 1(1), pp. 9-11, (1991); Nee, C. C. and Fuchigami, L. H., "Overcoming Rest at Different Growth with Hydrogen Cyanimide," Scientia Horticulturae, 50, pp. 107-113 (1992); and North, M. S., "Alternative Rest-Breaking Agents to DNOC/oil for Apples," S. Afr. J. Plant Soil, 9(1), pp. 39-40 (1992).
Hydrogen cyanamide is a skin and eye irritant and is especially acute when used in combination with the consumption of alcohol. The toxicity and relatively high price limit its market acceptance and hydrogen cyanamide has also been placed on the European Red List.
Thus, there exists an immediate need for new, milder rest-breaking agents, not only in South Africa, but also in countries which are currently developing agricultural industries but do not have an ideal climate therefor, such as Brazil. Further, there is a need for such new rest-breaking agents in countries with existing agricultural industries that, up to now, did not appreciate the extent of the problem of delayed foliation.
The response of a plant to a rest-breaking agent is dependent upon the chemical composition of the agent, its application rate and timing and on the nutritional status of the plant according to Terblanche, J. H. and Strydom, D. K., "Effects of Autumnal Nitrogen Nutrition, Urea Sprays and a Winter Rest-Breaking Spray on Budbreak and Blossoming of Young `Golden Delicious" Trees Grown in Sand Culture," Deciduous Fruit Grower, 23 pp. 8-14 (1973).
Many chemicals have been found to have rest-breaking ability. A summary of some of these chemicals can be found in Saure, M. C., "Dormancy Release in Deciduous Fruit Trees," Janick, J. (Ed.), Horticultural Reviews, 7, pp. 239-287, AVI Publishing Co. Inc., Westport, Conn. (1985). The efficacy of these various rest-breaking agents appears to be directly related to their physiological harshness.
One of the milder rest-breaking agents is potassium nitrate which has been shown to have a positive effect on peaches. Generally, fruit trees having a lower chill requirement, such as peaches, also require lower concentrations of rest-breaking agents, whereas fruits with a higher chill requirement, such as apples, require excessive concentrations of a mild rest-breaking agent. Articles demonstrating the effects of thiourea, potassium nitrate and combinations thereof include, Wolak, R. J. and Couvillon, G. A., "Time of Thiourea-KNO.sub.3 Application on the Rest Requirement and Bud Development in `Loring` Peach," HortScience, 11(4), pp. 400-402, (1976) and Fernandez-Escobar, R. and Martin, R., "Chemical Treatments for Breaking Rest in Peach in Relation to Accumulated Chilling," J. Hort. Sci., 62(4), pp. 457-461, (1987).
For the foregoing reasons, there exists a need in the art for improved rest-breaking agents which are, affordable, effective, less toxic than the present rest-breaking agents and which can be employed in reasonable concentrations.